1. Field of the Invention
The invention relates to the formation of polymer films, including highly conducting and transparent thin polymer films.
2. Description of the Prior Art
Extended π-conjugated conducting oligomers and polymers have unique properties that have impacted diverse technologies, and have resulted in the appearance of new ones. A partial list of current and developing applications includes micro- and nanoscale circuitry, throwaway electronic devices such as plastic electrochromic displays, flexible displays, lightweight storage batteries, corrosion protection coatings, antistatic coatings, bio- and chemical sensors, and military applications such as microwave-absorbing materials. In all of these applications a high degree of polymer transparency in visible wavelengths is either necessary or could represent an additional advantageous trait.
Key properties of π-conjugated conducting oligomers and polymers such as bandgap, dielectric constant, and oxidation potential can be varied over much wider ranges than those of transparent inorganic conductors such as indium tin oxide (ITO) ceramic. This is because of the vast diversity inherent to the organic chemistry of π-conjugated monomers. Other advantages over metals and inorganics include greater plasticity and elasticity, lower mass density, lower coefficient of thermal expansion, greater resistance to chemicals and corrosion, electrochromism, and enhanced power storage capabilities.
As an example of a specific application wherein a highly transparent conducting polymer could have a large impact, one can consider the liquid crystal display devices (LCDs) that are extremely important in current information technology and organic light emitting diodes (OLEDs) under development for next generation displays. In these devices, or in any display device, transparent electrodes are a prime requirement and ITO coated on glass (or more recently, clear plastic) surfaces has generally been used up to now because of its high transparency (˜90%), low surface resistance (˜70 ohms/sq), and high conductivity (˜1000 S/cm). However, the technology is quite expensive and requires high temperature and vacuum treatment. Moreover, the brittleness of the ITO, the non-stoichiometric nature of ITO surfaces, and poor adhesion at the inorganic-organic interface causes serious problems. The deposition of transparent, conductive polymer film on plastic substrates is a highly promising alternative that allows circumvention of these problems.
Because conducting oligomers/polymers are highly conjugated, they are colored both in the neutral undoped (non-conducting) state as well as in the cationic, doped (conducting) state. The development of highly transparent conducting polymer thin films has therefore been challenging. Prior art has centered on three families of conducting polymers, polyaniline (PANI), polypyrrole (PPY), and poly(3,4 alkylenethiophene) (PATP). PANI films formed by spin-coating the polymer onto clear plastic (polyethylene terepthalate or PET) had a surface resistance of 166 ohms/square and a transparency of ca. 82% over the range 475 to 675 nm, but the transparency sharply decreased below 475 nm giving the film a strong yellowish cast (Cao et al., “Optical-Quality Transparent Conductive Polyaniline Films,” Synth. Metals, 1993, 57, 3526, incorporated herein by reference). Composite films based on transparent Nylon-6 impregnated with PANI had conductivities of 10−2 S/cm with optical transparencies of 75%. Analogous films impregnated with PPY had conductivities of 10−3 S/cm, again with optical transparencies of 75% (Im et al., “Preparation and Properties of Transparent and Conducting Nylon 6-Based Composite Films,” J. Appl. Polym. Sci., 1994, 51, 1221, incorporated herein by reference). An additional composite film based on polyvinylchloride and PPY had a conductivity of ˜20 S/cm with an optical transparency of ˜53% (Kang et al., “Preparation and Morphology of Electrically Conductive and Transparent Poly(vinylchloride)-polypyrrole Composite Films,” Polym. Bull., 1993, 31, 593, incorporated herein by reference). Similar results were attained using PATPs and their oligomers. For example, a complex of oligo(3,4 ethylenedioxythiophene) and poly(styrene sulfonate) marketed as Baytron P by Bayer Fine and Specialty Chemicals (Leverkusen, GE) yields films with a surface resistance of ˜2600 ohms/sq, a conductivity of ˜5 S/cm and a transparency of ca. 80% when spin-coated from aqueous solution (Kumar et al., “Conducting Poly(3,4-alkylenedioxythiophene) Derivatives as Fast Electrochromics with High-contrast Ratios,” Chem. Mater., 1998, 10, 896, incorporated herein by reference). PATPs with alkylidene groups such as ethylene, propylene, and butylene as well as those containing phenyl and tetradecyl moieties yielded films with modest properties when formed via electropolymerization. For example, poly(3,4 ethylenedioxythiophene) films had a conductivity of 8 S/cm with a transparency of 52% (Jonas et al., U.S. Pat. No. 5,792,558, incorporated herein by reference).
An attractive attribute of the monomeric alkylidenethiophenes is their low oxidation potential (˜0.4 V relative to Ag/AgCl) that allows use of mild oxidation agents and results in polymer with high chemical stability. The polymers also have a low band gap (1.5–1.6 eV), causing their absorption λmax values to appear at relatively long wavelengths (590 nm for the undoped form and 775 nm for the doped form). The corresponding colors are dark violet and sky blue. The absorption in the doped conducting form is shifted into the infrared region and therefore the polymers become less heavily colored and are more transparent to the human eye. Within this class of conducting polymers, by far the most extensively investigated has been poly(3,4 ethylenedioxythiophene), or PEDOT, the simplest one from the standpoint of chemical structure (FIG. 1).
A polymerization method that is well suited for monomers with low oxidation potential such as PEDOT utilizes an oxidant, iron (III) p-toluenesulfonate, in combination with imidazole as a moderator in an alcohol solvent. At moderately high temperatures (˜100° C.) the polymerization occurs very rapidly. If the reactant-containing solution is spin-coated onto a suitable substrate such as plastic or glass and then heated, highly conducting insoluble sky-blue films are formed (De Leeuw et al., “Electroplating of Conductive Polymers for the Metallization of Insulators,” Synth. Metals, 1994, 66, 263, incorporated by reference) with reported conductivities of 300 S/cm.
The transparency of conducting polymers nominally follows Beer's law (A=εct) where A is the total absorption, ε is the molecular absorption, c is the concentration of the absorbing species, and t is the path length (thickness of the sample). Making thinner films will result in higher transparency, but generally leads to higher resistances. Moreover, the upper limit of transparency is dictated by the material itself due to the molecular absorption for different conducting polymers. Therefore, materials with low molecular absorption and high conductivity are required for the desirable combination of high transparency and low resistance.